Abstract
ABSTRACT The aim of present contribution is to describe the osteohistology of an articulated skeleton of an unnamed elasmosaurid coming from Santa Cruz province, Argentina, and recovered in late Cretaceous beds of Calafate Formation (Maastrichtian). Histological analysis indicates that the specimen reached somatic and sexual maturity. The specimen exhibits a unique combination of histological features that suggest complex palaeobiological and palaeocological implications. One remarkable aspect of the specimen is the presence of an external fundamental system (EFS) in the gastralia, a feature only cursorily described for plesiosaurs. Occurrence of EFS indicates a somatic maturation for the specimen. The transverse section of the phalanx and vertebral apophysis are osteoporotic, whereas those of the dorsal rib and gastralia show two layers of thick pachyostosic cortical tissue surrounding osteoporotic bone. This contradicts previous hypotheses that proposed that in elasmosaurids, the juveniles (characterised by osteoporotic histology) inhabited coastal waters and migrated to the open sea after reaching adulthood after an ontogenetic shift in bone density.
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